It is known to provide optical devices of an integrated type, i.e. wherein lenses and other optical components are formed by compositional modifications of regions of an optical waveguide such as a cut or section of an LiNbO.sub.3 crystal.
As for classic three-dimensional optical geometry, integrated optics utilizes means for implementing optical components which are related to the availability of materials or technologies which can vary the refractive index of the waveguide along the optical transmission path.
In one approach for the formation of integrated optical elements or components in a LiNbO.sub.3 planar waveguide, a proton exchange (Li.sup.+ /H.sup.+) is effected by contacting a selected region of the planar waveguide with molten (fused) benzoic acid at a temperature and for a period of time sufficient to allow the proton exchange to be carried out to the desired degree.
It has been found that the benzoic acid treatment, even when the melt is diluted with lithium benzoate, is not always satisfactory because the treated region does not have sufficient thermal stability, for example, to allow further modification as may be desired if, in a previously modified region having a refractive index different from that of the original waveguide, it is desired to form a more limited region with still further refractive index variations.
Because of the insufficient thermal stability of the first region, modifications which involve thermal treatments cannot be carried out effectively without damage to the original refractive index profiles which are detrimental to the integrated optical system.